Add Mcp-Session-Id header to HTTP MCP server requests

[Copilot]

HTTP MCP servers require the Mcp-Session-Id header for request authentication, but the gateway was not propagating it, causing 400 errors from servers like safeinputs.

Changes

• Connection layer: Modified SendRequestWithServerID to accept context.Context and extract session ID from context to include as Mcp-Session-Id header in HTTP requests
• Context key management: Defined SessionIDContextKey in internal/mcp package to avoid circular dependencies; re-exported in internal/server for backward compatibility
• Call sites: Updated all invocations in unified.go and server.go to pass request context through to connection layer

Implementation

// Extract session ID from context and add header
if sessionID, ok := ctx.Value(SessionIDContextKey).(string); ok && sessionID != "" {
    httpReq.Header.Set("Mcp-Session-Id", sessionID)
}

The session ID flows from the HTTP request → context → connection layer → HTTP backend, enabling proper authentication with HTTP MCP servers.

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